* basic-block.h (struct loop): Delete fields pre_header_root
	and pre_header_trace and replace with pre_header_edges
	and num_pre_header_edges.
	* flow.c (flow_loop_dump): Dump pre_header_edges.
	(flow_loops_free): Free pre_header_edges.
	(flow_loop_pre_header_scan): Calculate pre_header_edges.

From-SVN: r36934